This invention relates to monitoring the reactor core in a nuclear power plant and, particularly, to arranging gamma thermometers in the core of a boiling water nuclear reactors (BWR).
A typical BWR nuclear power plant includes nuclear instruments that monitor the condition of the reactor core. The signals generated by these instruments are used to maintain the reactor core within allowable operating conditions. The instrument signals may be processed by a core monitor software that determines the 3-Dimensional (3D) nodal powers and the 2-Dimensional (2D) bundle flows. The 3D nodal powers and 2D bundle flows may then be used to determine thermal margins within the reactor core. The operators may use the determined thermal margins to make adjustments to the core operating conditions so not to exceed the allowable operating conditions. Further, the 3D nodal powers and 2D bundle flows may be used by plant operators to confirm that the reactor core is operating within allowable operating conditions.
The instruments for a typical BWR nuclear reactor include a Transverse In-core Probe (TIP) system and a Local Power Range Monitor (LPRM) and/or Start-Up Range Neutron Monitor (SRNM) systems. The available types of TIP instruments comprise instruments to measure neutron thermal flux and instruments to measure gamma flux. LPRM instruments generally measure neutron thermal flux.
TIP and LPRM instruments are arranged in a core to take axial measurements at fixed radial locations in the core. Conventionally, the TIPs are mechanically moved in and out of the core to calibrate the LPRMs and, particularly, to calibrate individual detectors in each LPRM. During the calibration process, a TIP is positioned next to a detector of a LPRM and the LPRM gain electronics are adjusted to cause the LPRM detector to generate an output signal equivalent to an output signal from the adjacent TIP. In addition, the TIPs may provide processed output signals indicating the neutron thermal flux and gamma flux at various elevations in the reactor core, such as at elevations at six inch (15 centimeters) intervals. The flux measurements taken at various elevations of the core provide axial information regarding the power shape in the core at non-LPRM core locations.
Maintaining and operating the TIP mechanical system to raise and lower the TIPs is expensive. Gamma Thermometers (GT) sensors have been used instead of TIPs. Unlike the TIPs that were moved in and out of the core, the GT sensors are positioned at fixed axial locations in the core. Similar to TIPs, the GT sensors are used to calibrate the LPRMs. Because the GT sensors are at fixed axial locations, the expense of a mechanical movement system to raise and lower the TIPs has been eliminated for the stationary GT sensors.
In a conventional application, seven or more GT sensors are arranged as a linear array, such as on a vertical string. These vertical arrays of seven GT sensors are positioned at various fixed elevations in the reactor core. The fixed elevations for the GT sensors are manufacturing determined and correspond to fixed axial positions on the string supporting the GT sensors.
Fabricating the GT sensors on the strings is problematic due to the narrow tolerances for axial placement of the GT sensors in each string. Each GT of a string must be positioned precisely on the string to be positioned in the core at the elevations to which they are assigned. The GT sensors in each vertical string are each positioned within narrow vertical tolerances to assure that each GT sensor is positioned at its assigned axial position, e.g., adjacent a LPRM when the GT sensor is placed in the core. The narrow vertical tolerances for the GT strings are necessary so that the LPRMs can be accurately calibrated. Each string of GT sensors is permanently fixed in the core after the array is properly positioned and vertically aligned with the LPRMs.
To expand the vertical tolerances for the GT strings would, in combination with the inherent uncertainty in any nuclear measurement systems, e.g., the LPRMs, create uncertainties in the determination of the operating conditions of a reactor core. An increase in the uncertainties in the determination of core operating conditions will likely lead to a narrowing of the reactor core operating limits as the operating margins are increased to compensate for the increased uncertainties. The increase margins can result in additional reactor fuel cost as the acceptable operating conditions are narrowed.
The narrow axial tolerances applied to the strings of GT sensors are problematic with respect to the manufacturing of these arrays. The tolerances reduce the number of GT sensors that can be accurately positioned on a GT string to, for example, seven GT sensors. The limited number of GT sensors that can be manufactured on each string reduces the amount of core information that can be sensed by the GT sensors.
The amount of information regarding the axial power shape of a core that can be sensed by a GT string is dependent on the number of GT sensors vertically arranged on the string. Each GT sensor collects data at a particular axial position on the string, which corresponds to an elevation in the core. Limiting the number of GT sensors on each GT string limits the core elevations for which there is data from GT sensors.
The amount of information regarding the axial power shape increases as the number of GT sensors on a string increases. For example, seven GT sensors on a string provides less information regarding the axial power shape at various core elevations than would twenty GT sensors on a string. Increasing the information that is sensed by the GT sensors regarding the axial power shape in a core reduces the uncertainty of that power shape. A reduction in the uncertainty of the power shape allows for a corresponding reduction in the margins applied to the core operation limits. Reducing the uncertainty margins, allows for the core to be operated at conditions that are safe and more efficient respect to fuel consumption.
It is conventional for the axial locations of each GT/sensor in a string to be specified prior to the manufacture of the string. In addition, each GT string for a core is manufactured such that the GT sensors are arranged at the same axial locations on each string. Thus, GT sensors are arranged at the same core elevations for every axial location of the GT arrays. Because these GT elevation locations are fixed, the GT elevations are typically hard coded into the core monitoring software. The core monitoring software does not allow for GT sensors to be arranged at core elevations outside the assign axial positions for each sensor and the narrow tolerances predefined for the GT sensors.
There is a long felt need for an arrangement of GT sensors that can be readily manufactured and provide an increased amount of information regarding the axial power shape of a core.